Aviation turbine fuels (also known as jet fuels) are used for powering turbine engine aircraft. One of the key performance properties of jet fuels is their fluidity measured in terms of freezing point and viscosity.
For powering civilian or commercial aircraft, there are two main grades of jet fuel: Jet A-1 and Jet A. Jet fuels of both grades are kerosene-type fuel and the difference between them is that jet fuel of grade Jet A-1 fulfills the freezing point requirement of maximum −47° C. whereas jet fuel of grade Jet A fulfills the freezing point requirement of maximum −40° C. There is another grade of jet fuel: Jet B for usage in very cold climate. Jet fuel of grade Jet B is a wide-cut fuel covering fractions from naphtha and kerosene, which fulfills the freezing point requirement of maximum −50° C.
Greener jet fuels are being sought for replacing the existing petroleum-based jet fuels. Several alternatives have been considered for this purpose. The alternatives considered are alcohols including methanol and ethanol; cryogenic fuels including hydrogen and methane; and biofuel including vegetable oils and methyl esters derived from vegetable oils. For feasible replacement of the existing petroleum-based jet fuels, such alternatives must be compatible with the quality requirements of existing aircrafts, for example, they must have sufficient energy content and adequate lubricity and they must also be compatible with all the materials used in the engine's fuel system.
Alcohols are impractical as jet fuel because of their low volumetric energy content and their chemical incompatibility with materials used in the engine's fuel system. Furthermore, alcohols have very low flash point making them very hazardous to be used.
Cryogenic fuels are not compatible with the fueling system of existing aircrafts. Introduction of any cryogenic fuel as jet fuel would require the design and development of new aircraft as well as new supporting airport infrastructure for the storage and handling of such fuel. Cryogenic fuels also have low volumetric energy content making it necessary for the new aircraft to have larger fuel tank than existing aircrafts to take up a larger fuel load.
Although vegetable oils have the highest volumetric energy content among the alternatives considered but they are totally unsuitable to be used as jet fuel because they cannot meet the freezing point requirement. Even when methyl esters derived from vegetable oils are used, they can only be added to jet fuel in an amount of not more than 2%. If they are added in an amount of more than 2%, the resultant fuel blend would fail the freezing point requirement.
Till present, there is no feasible alternative for the existing petroleum-based jet fuel.